(1) Field of the Invention
The present invention relates to a diagonal flow fan.
(2) Description of Related Art
Heretofore, various methods for reducing the noise of the fans have been proposed, and for example, there are some fans disclosed in Patent Document 1 (JP-A-11-201099) and Patent Document 2 (JP-A-2001-115821). Any of these fans is configured to reduce the noise by a silencer. The Patent Document 1 discloses a fan having a sound absorbing structure on the upper stream side of the fan for diminishing the noise and sucking not from an axial direction but from a radial direction. The Patent Document 2 discloses an air blower having a cylindrical main body configured to suck from a radial direction, in which noise absorbing part bodies are disposed in the suction port directly opposite one another retaining predetermined spacing.
In general, a diagonal flow fan is characterized in that, comparing with a centrifugal fan, the air pressure is low and the quantity is much, and comparing with an axial flow fan, the air pressure is high and the quantity is little, whereas the diagonal flow fan targeted by the present invention is designed such that a specific speed Ns in the vicinity of the best efficiency point defined by the following formula (1) is 800 to 1400.Specific speed Ns=N×Q0.5×H−0.75  Formula (1)                N: number of revolutions (min−1)        Q: quantity (m3/min)        H: head (m)=pressure/acceleration of gravity g/γ        γ: fluid density        
This is a dimensionless quantity issuing a guide line for designing whether it is the pressure type or the quantity type when the standard state of the air is 1.2 kg/m3 and the specific speed Ns is considered based on the outer diameter of the same impeller, and shows that the higher the specific speed Ns is, the more the quantity is. Hence, a flow noise of the flow becomes a main component in the noise of the diagonal flow fan. That is, when a frequency analysis is conducted, the component of frequency of 500 to 1000 Hz (hereinafter, this frequency range is referred to as moderate frequency range) becomes loud. One example of the noise characteristic of the diagonal flow fan in which the specific speed Ns is 1000 is shown in FIG. 4.
In the technique disclosed in the Patent Document 1, when the silencer is configured to suck from a radial direction, a fan casing rather than a suction port becomes larger in the centrifugal fan, which is required to provide a scroll or a fluid recovery flow channel around the impeller. Consequently, though the entire configuration does not become large by the configuration itself of the silencer, in the case of the diagonal flow fan, since the suction port and the fan casing have approximately the same size in outer shape, there has been a problem that the outer shape of the entire fan becomes large due to the configuration of the silencer.
In the Patent Document 1, sound absorption of the noise depends on the characteristic and the surface area of a sound absorbing material as well as the shape such as a thickness. The sound absorbing characteristic of the sound absorbing material can obtain high sound absorption coefficient in the high frequency of or above 1000 Hz, whereas in the case of a fan having specific speed Ns of 800 to 1400 such as the diagonal flow fan, rather than the noise of the high frequency range, the noise of the moderate frequency range is liable to be generated, and there has been a problem that the noise is not sufficiently absorbed.
That is, in the diagonal flow fan, since a pressure interfered sound with the fan casing is low in noise characteristic, the noise of the moderate frequency range of 500 to 1000 Hz generated by the turbulence of the flow at the suction time becomes loud, and there has been a problem that the noise in this moderate frequency range is required to be reduced.
Similarly in the technique disclosed in the Patent Document 2, though the silencer is configured to suck from the radial direction, it is configured to depend on the sound absorbing material similarly to the Patent Document 1. That is, the air blower of the Patent Document 2 is not the diagonal flow fan, and for this reason, it is different in the noise characteristic, and does not give consideration to the noise of the moderate frequency range.